Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2000-01-03
2001-07-10
Zweizig, Jeffrey (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S563000
Reexamination Certificate
active
06259300
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a differential input interface circuit and to a method for adjusting DC levels of differential input signals. More specifically, it relates to a differential input interface circuit that matches the DC levels of the differential input signals to the DC level of a system which is responsive to the differential inputs, and to the method for adjusting the DC level of the differential input signals.
BACKGROUND OF THE INVENTION
An A/D (analog-to-digital) converter
100
is shown in FIG.
5
(
a
) as an example of a system responsive to differential inputs. The A/D converter
100
converts differential analog inputs, i.e., a non-inverting phase signal (IN(P)) and inverting phase signal (IN(N)) into a digital output signal (OUT). In such conversion of differential inputs, it is necessary to match the DC levels of the external analog input signals IN(P) and IN(N) (VDCP, VDCN) to the DC reference level (Vref) within the A/D converter
100
.
FIG.
5
(
b
) shows an example of prior circuits used for solving the problem of DC offset, wherein the conventional differential input interface circuit
102
consists of level shift circuits
104
and
1
14
, and buffer circuits
106
and
116
. The DC levels of the OUT (P) and OUT (N) (VDCP and VDCN) are adjusted by level shifting circuits
104
and
114
to match the DC level within the A/D converter
100
, even if the DC levels of the external inputs IN(P) and IN(N) fluctuate. In the event wherein the analog input is part of a single line structure, the DC level of the input signal needs only to be shifted to the DC level (Vref) at which a correct A/D converter operation must operate. In the case where the analog input belongs to a differential line structure, the DC levels of the input signals are required to coincide to each other and, in addition, to the DC level (Vref) within the A/D converter
100
. In other words, when there are differences between the DC levels of the analog inputs IN (P), IN (N) (VDCP, VDCN) and the DC level (Vref) within the A/D converter
100
, as shown in FIG.
6
(
a
), the DC level (VDCP) of the non-inverting phase signal OUT (P) is required not only to match the DC level (VDCN) of the inverting phase signal OUT (N), as shown in FIG.
6
(
b
), but also to the DC level (Vref) within the A/D converter, as illustrated in FIG.
6
(
c
).
FIG.
5
(
c
) illustrates another type of conventional differential input interface circuit
112
, consisting of capacitors
108
and
118
. This input interface circuit
112
cuts-off the DC levels of the differential analog inputs IN (P) and IN (N) by way of AC coupling, using capacitors
108
and
118
for this purpose. The removal of the DC level eliminates the adverse effect of the DC offset (i.e., level fluctuation) both external to the A/D converter and from the interface. However, in this case, it is required to create an appropriate DC level within the input interface circuit
112
to ensure a correct operation of the A/D converter
100
. This must be done simply and easily. In this manner, in order to eliminate the adverse effect of the offset from outside the A/D converter and the interface, it is required that the DC level of the external inputs match the correct operation level within the A/D converter.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a differential input interface circuit which eliminates the problem of a DC offset between differential inputs without resorting to level-shifting, and to a method for adjusting the DC levels of the differential input signals.
It is another object of the invention to precisely match the DC level of the differential input signals to a reference DC level.
It is still another object to use the reference level Vref generated within the system as the DC level for the differential inputs, thereby eliminating the requirement for additional operations to process the DC levels per se originally contained in the differential inputs.
It is a further object of the present invention to simplify the design of differential input interface circuits while improving the reliability of matching the DC levels.
In accordance with a first aspect of the invention, the differential input interface circuit used within a system that is responsive to differential inputs includes a reference level generation stage for generating a DC level identical to the DC level within the system, and a DC level matching circuit for matching the DC levels of the differential input signals to the DC level of the reference level generation stage.
In a second aspect of the invention, there is provided a method for adjusting the DC levels of differential input signals which includes the steps of: cutting-off the DC component of the differential input signals, and superimposing the differential input signals from which the DC component was cut off, on the DC level prevalent throughout the system.
REFERENCES:
patent: 4152659 (1979-05-01), Gordon
patent: 4320351 (1982-03-01), Brown, Jr. et al.
patent: 5170081 (1992-12-01), Adachi et al.
patent: 5300896 (1994-04-01), Suesserman
patent: 6100759 (2000-08-01), Sirna et al.
Hatani Naohisa
Yasuda Takeo
International Business Machines - Corporation
Schnurmann H. Daniel
Zweizig Jeffrey
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